Thermal Stability and Melt Processing Rules for Polyhydroxyalkanoate Resins
Polyhydroxyalkanoate processing demands strict moisture control below 200 ppm and residence times under three minutes to prevent cis-elimination chain scission.
Secondary Spherulite Growth
Meaning
Post-moulding crystallization processes occur over time within the amorphous regions of semi-crystalline polymers after the primary solidification from the melt has finished. This secondary spherulite growth occurs as the polymer chains slowly reorganize into denser crystal structures. The ongoing rearrangement increases the density and stiffness of the polymer part.
Dimensional Change
Shrinkage and warping can occur long after the part has been ejected from the mould. This post-moulding reorganization caused by secondary spherulite growth alters the part dimensions, leading to a risk of assembly failure. Moulders must account for this delayed shrinkage when designing tooling.
High mould temperatures during processing speed up the primary crystallization, which reduces the amount of delayed growth that occurs later.
Mechanical Alteration
Tensile strength and modulus increase while impact resistance and ductility decrease as crystallization continues. If secondary spherulite growth continues unchecked, the moulded part becomes brittle over time. This brittleness is particularly problematic for components that must absorb impacts or undergo flexing during use.
Monitoring the crystal structure allows moulders to predict the long-term performance of the plastic.
Mitigation Strategy
Annealing the moulded parts at elevated temperatures completes the crystallization process quickly. This thermal treatment ensures that the part dimensions and mechanical properties stabilize before the components are shipped.